1. Field of the Invention
The present invention relates to a method of phase rotation for a wireless communication system, and more particularly, to a method of phase rotation for reducing a signal PAPR in the wireless communication system.
2. Description of the Prior Art
Wireless local area network (WLAN) technology is one of popular wireless communication technologies in the world. In the beginning, WLAN technology is developed for military use, while in recent years, WLAN technology is widely implemented in consumer electronics, e.g. desktop computers, laptop computers, personal digital assistants, etc., to facilitate convenient and high-speed wireless communication. IEEE 802.11 standard is a set of WLAN protocols established by the Institute of Electrical and Electronics Engineers (IEEE). Products complying with IEEE 802.11 standard are authenticated by the Wireless Fidelity Alliance (WFA) and then offered a trademarked brand name WiFi after passing the authenticated procedures of the WFA.
In details, IEEE 802.11 is composed of more than 20 different standards distinguished from each other by a letter appended to the end of IEEE 802.11. The familiar IEEE 802.11 series is IEEE 802.11a, 802.11b, 802.11g, 802.11n standard and so on. The most difference among each of the IEEE 802.11 series is modulation method and maximum data rate. For example, for modulating signals to be transmitted, IEEE 802.11a/g/n standard utilize orthogonal frequency division multiplexing (OFDM) method, whereas IEEE 802.11b/g utilize direct-sequence spread spectrum (DSSS) method. IEEE 802.11n standard is different from IEEE 802.11a/g standard in adding a multiple-input multiple-output (MIMO) technique and other features that greatly enhance data rate and throughput. In addition, in IEEE 802.11n standard, a channel bandwidth is doubled to 40 MHz from 20 MHz in IEEE 802.11a/g.
OFDM method has advantages of high spectrum utility efficiency and capability of resisting signal attenuation caused by a multi-path propagation. After a transmitter in WLAN systems modulates signals to be transmitted by OFDM method, a peak to average power ratio (PAPR) of modulated signals may easily be excessively high, and a distortion may occur when the modulated signals are processed in radio frequency (RF) circuits of the transmitter. Afterwards, a packet detection rate in a receiver may be decreased.
In order to reduce the PAPR of modulated signals, a method of phase rotation is introduced in IEEE 802.11n standard. First, a 40 MHz channel for transmitting modulated signals in IEEE 802.11n standard is divided into two 20 MHz subchannels, which are an upper 20 MHz subchannel and a lower 20 MHz subchannel in frequency domain. Afterwards, a phase rotation of 90 degrees is applied to the upper 20 MHz subchannel so that the PAPR of the 40 MHz channel in IEEE 802.11n standard is reduced when transmitting packets with modulated signals. Thus, the packet detection probability in a receiver is improved.
Furthermore, for achievement of a higher quality WLAN transmission, the IEEE committee establishes a new generation IEEE 802.11ac standard, which is IEEE 802.11 VHT (Very High Throughput) standard. Compared to IEEE 802.11n standard, IEEE 802.11ac also utilizes MIMO technique, but doubles a channel bandwidth to 80 MHz. Moreover, IEEE 802.11ac standard may further provide a solution of a 160 MHz transmission to greatly enhance data rate and throughput. Possibly, the 160 MHz transmission may be composed of two 80 MHz channel segments with a frequency separation from each other, which is called a non-contiguous channel configuration. Due to bandwidth increase and the non-contiguous channel configuration in IEEE 802.11ac, the method of phase rotation in IEEE 802.11n standard cannot be applied directly into 802.11ac standard. Therefore, how to reduce PAPRs in IEEE 802.11ac standard is necessary to develop.